SRS iWOW is one of the best things I've ever had on my Mac, after a quick and super easy installation it really improves the sound quality to a level that your ears won't believe.
Support is also top notch (though I needed their help only once as it's rock solid) !
I tried some other sound improvement software (like Hear) and must say that I reverted to SRS iWOW after just a few minutes.
First, others do not support external audio hardware like my beloved Creative X-FI (sound christalyser).
Second, others aren't as straightforward, too many settings may confuse even experienced users. And, like me, you may well experience problems with other software that use audio (Skype etc...)
Bottom line :
The few extra bucks you'll pay for SRS iWOW over Hear are well worth it !
As far as I'm concerned I highly recommend iWow, all my audiophiles friends say the same !